Semiconductors, such as integrated circuits, are utilized in a wide variety of electronic devices, such as pocket calculators and laptop computers. Typically, semiconductors are formed on wafers which are cut into discs or chips that individually may be mounted on substrates. Typically, an integrated circuit is attached to a substrate by means of a bond that provides both electrical and thermal conductivity between the circuit and the substrate.
Known methods for making an electrically and thermally conductive bond between a die and a substrate include: employing a solder or eutectic alloy, such as a gold-silicon alloy; employing a spreadable adhesive consisting of heat-curing epoxy resin composition filled with fine metal particles; and employing an electrically and thermally conductive adhesive composition which comprises an adhesive containing fine metal particles or a deformable metal foil. See, for example, U.S. Pat. No. 4,606,962.
The metal eutectics are used most specifically in the area of power devices, to provide a metallurgical interface between a silicon die and the heat-sinking metal or ceramic substrate with optimum thermal and electrical conductivity. This technique is relatively successful for smaller devices, but is not desirable for use with larger dice, e.g., 1.5 cm on a side. The differential coefficients of expansion of the substrate and the silicon die can result in a larger die cracking under the stresses imparted by a very rigid bonding medium, and may result in its subsequent failure.
Epoxy-silver compositions are used extensively for commercial die-bonding, as they provide an often suitable compromise in terms of cost, stress-relief, and electrical/thermal conductivity. However, epoxy-silver compositions have the following undesirable characteristics: the lack of uniformity of dispersion of silver particles within the adhesive composition, the lack of uniformity of the mixture of two component systems (epoxy and curative), the lack of coplanar (die/substrate) maintenance during cure, the presence of resin bleed into the surrounding area or onto the die's active surface prior to curing, and unsuitably low shear strengths, as measured by the military standard, MIL-883C.
A desirable alternative bonding means is a conductive adhesive. Ideally, a conductive adhesive is provided as a self-supporting film. Prior art liquid epoxy systems require constant mixing to maintain a suitable dispersion of silver or other conductive particles, and must be dispensed in excess to assure that the entire die bonding surface is wet during die placement. These liquid systems by their nature will migrate due to capillary action and may contaminate or cover critical areas of the die or substrate in which no adhesive may be tolerated.
In contrast film is capable of being cut with the wafer to the precise size of the die. This provides the exact amount of adhesive in the precise area necessary for die bonding. Flow of this adhesive is very limited, and may occur only at the time of bonding, when flow is desired.
Adhesive films can also be used for establishing multiple, discreet electrical interconnections between two substrates. For these applications, the adhesive film may contain sparse populations of fine conductive particles, which provide electrical conductivity through the thickness of the film, but not in the plane of the film (anisotropically conductive). The use of a film-type adhesive for these applications provides the possibility of using either a random or a uniform particle distribution, which is a choice that is not available when using paste or liquid systems. Because of their anisotropic conductivity, these materials are often referred to as Z-axis Films (ZAF).
Unlike solder interconnections, ZAFs provide pressure-engaged interconnections. The role of the adhesive is to establish and maintain normal forces at each contact interface in order to provide stable and low-resistance interconnections. The source of these normal forces is thermal stress which builds during cool down from the bonding temperature and which builds during cool down from the bonding temperature and which is the direct result of thermal expansion mismatch between the adhesive and the conductive particles.
An adhesive that is used as an electrical interconnection medium must be in direct contact with a portion of the active circuitry. Thus, another important requirement of the adhesive is that it protect the contacted portion of the active circuitry from galvanic corrosion processes. In order to do this, it is important that the adhesive be essentially free of extractable ionic impurities, and it is also desirable that the adhesive possess low moisture absorption.
Despite the advantages achieved by using an adhesive interconnection, solder has continued to be the material of choice in many applications. Commercially available anisotropically conductive adhesive films have not demonstrated adequate performance under a sufficiently wide range of conditions to permit a more widespread use.
An improved anisotropically conductive adhesive, able to perform under a wider range of use conditions, is preferably relatively tack-free at room temperature to enable handling and repositioning of the undiced wafer prior to lamination. It is desirable that the adhesive film be capable of rapidly curing to effect bonding of a circuit to a variety of substrates. It is also desirable that the adhesive exhibit a long shelf life (i.e., is storage-stable); excellent creep resistance (substantially superior to solder); be essentially free from extractable ionic impurities, possess low moisture absorption; and is thermally curable to rapidly form an adhesive bond that exhibits superior shear strength, and adhesion to a multiplicity of substrates and surfaces.